This application is related to Japanese Patent Application No. 2001-244376 filed in Aug. 10, 2001, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a light emitting device and, particularly, to a light emitting device for a printer head.
2. Description of the Related Art
In recent years, display devices employing organic thin film electroluminescent devices have actively been developed by research institutes and corporations, and some of the display devices have already been put into practical use by some makers. The research and development are now directed not only to flat displays including display devices provided on a flat glass substrate but also to flexible displays including organic thin film electroluminescent devices provided on a plastic film and adapted to be installed on a curved surface of a pillar or to be rolled for transportation thereof.
On the other hand, laser beam printers employing a laser light source and LED printers employing an LED light source have been put into practical use.
Printer heads and printers employing organic electroluminescent devices as a light source are also under research and development, which are promising as less costly heads and energy-saving printers (see, for example Japanese Unexamined Patent Publications Nos. Hei 9(1997)-226171 and 2001-071558).
In the laser beam printer, a laser beam is turned on and off according to an image pattern, while being scanned over a photoreceptor by means of a polygon mirror. Therefore, a correspondingly greater space is required for optically scanning the laser beam from one end to the other end of the photoreceptor, making it difficult to reduce the size of the laser beam printer. Where a greater-size optical system is required for scanning a laser beam over a greater-size photoreceptor, the laser beam is incident on an edge portion of the photoreceptor at a greater angle, so that the laser beam is liable to be distorted.
The LED printer is easier in size reduction than the laser beam printer. However, LED chips of the LED printer suffer from variations in brightness, requiring measures against brightness unevenness, for example, screening of the LED chips and brightness adjustment by a driving circuit. This leads to an increase in costs.
On the contrary, the organic electroluminescent devices can collectively be formed on a film and, therefore, are advantageous for suppression of brightness variations, size reduction of the optical system and cost reduction when used for a printer head.
However, the printer head constituted by the organic electroluminescent devices has a lower light intensity per unit area, requiring application of a more intensive electric field for a higher electric current density than the display device to output light sufficient for exposure of the photoreceptor drum.
If the electric current density is increased, the organic electroluminescent devices tend to have a lower light emitting efficiency and a shorter service life. In order to reduce the light emitting intensity per unit area and, at the same time, to provide a sufficient light intensity on the photoreceptor, the area of each of the organic electroluminescent devices should be increased, and light beams emitted from the larger-area organic electroluminescent devices should be converged on the photoreceptor.
In view of the foregoing, the present invention is directed to a light emitting device which employs an organic electroluminescent device having a greater area and is adapted to converge a light beam emitted from the organic electroluminescent device into a sufficiently small light spot to provide a light intensity sufficient for exposure of a photoreceptor drum, and to a light emitting device array including a plurality of such light emitting devices.
In accordance with the present invention, there is provided a light emitting device, which comprises a lens member having a light inputting surface and a light outputting surface for converging light incident on the light inputting surface and outputting the converged light from the light outputting surface, and an organic EL device provided on the light inputting surface of the lens member.